PROJECT SUMMARY Characterization of a novel germline RNP granule defined by Tdrd5l One theme that has emerged rapidly over the past decade is that the ?common? cytoplasm and nucleoplasm of a cell can be subdivided into membraneless organelles formed by phase separation of interacting RNAs and proteins termed RNP granules. Specific types of RNP granules are associated with the germline, where they are important for germline specification (germ granules) and the piRNA pathway's ability to defend the germline from transposon activity (nuage). Through our work on germline sexual development, we have discovered that a new member of the Tudor-domain family, Tdrd5l, defines a novel germline RNP granule. Tdrd5l localizes to perinuclear granules that are associated with, but do not contain, nuage components such as the Vasa helicase. Thus, the ?Tdrd5l granule? is either a sub-compartment of the nuage or an entirely distinct granule. Consistent with this, Tdrd5l is not required for transposon regulation, as are components of the nuage. Interestingly, Tdrd5l has a unique localization to a sphere or shell surrounding the putative Tdrd5 granule. This localization is unusual for RNP granule proteins and suggests a role in stabilizing the granule, spatially segregating the granule from other RNP granules like the nuage, and/or acting as a gatekeeper for the granule. Our data indicate that Tdrd5l is important for different aspects of germline development, including sexual identity of the early, undifferentiated germline, and repression of maternal RNA translation in the differentiated female germline. In this proposal, we will study the protein and RNA components of the Tdrd5l granule and test the hypothesis that it controls post-transcriptional gene regulation by regulating mRNA translation.